A source of high energy gamma photons can be used to take radiographs (photographs using photons having higher energy than visible light) of metal structures such as castings or welds, similarly to the use of x-rays for radiographic imaging. Both techniques can be used to ascertain if flaws, defects, or cracks exist in a piece being tested without needing to dissect the piece to make a visual examination.
For industrial applications, the piece being tested might be a subsection or assembly intended as part of a larger critical appliance, for example a turbine blade. Alternatively, it might be a critical component in itself, for example a pipeline section. In each of these examples, failure of the component would have catastrophic repercussions and as such failure is not acceptable.
Although x-ray imaging can be used for certain applications, there are situations in which the use of x-ray is unsuitable, due to limitations on power supply or access, or due to ambient atmospheric conditions (the presence of flammable gaseous for example). It is often preferable to use radioisotopes for imaging, due to the increased flexibility and accessibility offered by this technique. A radiographic projector system can be used for radiographic imaging.
A radiographic projector is a device used to house a radioisotope used in the process of gamma radiography. The projector allows transportation and use of the radioisotope in a safe and reliable manner. For many years, the industry standard has been a remote windout system. This type of projector system can be remotely operated to project the radioisotope from the shielded, stored position inside the projector to the working position. In this way, a highly dangerous radioactive source can be manipulated from a distance, thereby minimising the exposure of the operator to harmful radiation.
The remote windout system comprises three main components: the projector, a windout, and guide tubing. The guide tubing can be connected to the front of the projector to guide the radioisotope to the working position. The windout can be connected to the rear of the projector and is commonly made up of a gear wheel with a handle for cranking, a control cable, cable housings for the control cable to reside or run in, and a connector for connecting the windout to the projector. In operation, the control cable is coupled to the radioisotope holder, and cranking the gear wheel causes the control cable to run through the cable housings to progress the source along a channel inside the projector and out of the projector through the guide tube. The windout can be operated at a safe distance from the projector, thereby allowing the radioisotope to be progressed from the projector remotely. The operator is exposed to a lower radiation dose due to operating the projector system from a large distance away.
Further accessories may be coupled to the projector system to increase the safety or flexibility of the system. For example, a guide tube may be connected to the front of the projector to guide the source to the work position when it is exposed by the windout system.
A problem associated with coupling shielded ancillary components such as collimators to the front of the projector is that radiation may escape unshielded through a gap between the collimator and the projector. This problem is known as hot passing.
EP 0 513 512 A2 discloses a gammagraphy apparatus including a locking mechanism for ensuring that a radioisotope remains locked inside a shielded body until a windout assembly and ancillary component are correctly attached to the body. Once these components are correctly attached, a locking slide can be moved to unlock the source holder. A spring biased locking block engages and holds the locking slide in the unlocked position until the radioisotope is safely returned to the shielded body. In this apparatus, the locking block is coupled to the movement of the source holder, such that the source holder is pushed forward by the locking block on moving the locking bar to the unlocked position. Winding the source holder back to its original position releases the locking block from the locking slide to lock the source holder in place.
A disadvantage of this type of mechanism is that, if the source holder cannot be pushed forward by the spring biased locking block, for example, due to an excessive frictional force associated with the attached windout cables, the interlock will not operate. In order to compensate the frictional forces, the spring value associated with the locking block is usually increased. However, this has the disadvantage of generating more wear in the lock and making assembly of the lock more difficult. More importantly, an increased spring value may be dangerous, because if the resistive force of the windout cable friction is not present, the source holder may be pushed forward from the stored position or perhaps even from the shielded container when an operator is in close vicinity of the projector.